1. Field of the Invention
The invention relates to a vehicular driving force control apparatus and control method of a vehicle having a powertrain that includes an engine and an automatic transmission. More particularly, the invention relates to a driving force control apparatus and driving force control method which appropriately outputs a driving force that corresponds to a driving force required by a driver.
2. Description of the Related Art
A control method known as “driving force control” has been employed in vehicles provided with an automatic transmission and an engine that can control engine output torque independently from an accelerator pedal operation by a driver. This driving force control calculates a positive and negative target driving force based on the accelerator pedal depression amount by the driver and operating conditions of the vehicle and the like, and realizes that calculated target driving force using engine torque and a speed ratio of the automatic transmission. Also, control methods referred to as “required driving force control” and “driving force demand control” are related to this control method.
A control apparatus of this kind of control method first calculates the target driving force to be generated in the vehicle based, for example, on the accelerator depression amount, the engine speed, external load, and a demand from a cruise control system. The control apparatus then converts the calculated target driving force into target torque taking the gear ratio of the automatic transmission and the like into account, and then controls the engine to generate this target torque.
According to the control apparatus of this kind of driving force control method, making the driving force which is a physical quantity that is directly applied to control of the vehicle a target value improves drivability by, for example, enabling a constant steering feeling to always be maintained.
Japanese Patent Application Publication No. JP-A-2005-180344 describes a drivetrain control apparatus in an integrated control system of a vehicle hierarchically configured such that calculations are performed from an upper control level which includes demands by the driver to a lower control level which includes actuators (actuators operate the transmission and drive source of the vehicle). This drivetrain control apparatus includes required output calculating means, target gear ratio determining means, shift controlling means, and generated torque calculating means. The transmission includes a torque converter, and the required output calculating means performs an inverse operation on the torque to be generated in the drive source from the required drive torque using a torque converter inverse model. The required output calculating means calculates the output required of the drive source. The target gear ratio determining means calculates the target gear ratio of the transmission. The shift controlling means controls the transmission, and the generated torque calculating means calculates the drive torque to be generated in the vehicle.
According to this drivetrain control apparatus, the necessary structure in the drivetrain control apparatus of the hierarchical integrated system of the vehicle is clear. That is, the required output calculating means calculates the required speed and required torque of the drive source. The target gear ratio determining means calculates the target gear ratio of the transmission. The shift controlling means calculates parameters that control the transmission. The generated torque calculating means calculates the drive torque to be generated in the vehicle. In this way, the necessary structure in the drivetrain control apparatus of the hierarchical integrated system of the vehicle is specified and the control parameters calculated by this drivetrain control apparatus can be output to an actuator of the lower control level and the control parameters can be output as availabilities to the upper control level. Accordingly, the required torque and required engine speed can be calculated by the drivetrain control apparatus based on various parameters related to the torque and vehicle speed that were calculated by the upper control level. In addition, the target gear ratio can be determined and the clutch load torque calculated from the shift duration and the target output shaft torque of the automatic transmission, and the control parameters can be output to an engine control apparatus and an automatic transmission control apparatus. More specifically, an inverse operation can be performed on the drive torque to be generated in the drive source from the required driving force using the torque converter inverse model. Therefore, in reality the drive source control parameters can be calculated from the required drive torque.
This torque converter is provided between an engine which serves as the drive source and, for example, a gear type stepped shifting mechanism. The output shaft of this shifting mechanism is connected via a differential gear to driving wheels. The torque converter is typically designed to ensure performance when the vehicle is in a driving state, i.e., when the input shaft rotation speed (on the engine side) is faster than the output shaft rotation speed (on the transmission side). Therefore, the model of the torque converter described above (and the inverse model which is the inverse function of this model) is also set to simulate the driving state which is the basic operating mode.
However, when the vehicle is in a driven state, i.e., when the output shaft rotation speed (on the transmission side) is faster than the input shaft rotation speed (on the engine side), the characteristics differ from those in the driving state. Therefore, the model of this driven state (and also the inverse model) differs from the model of the driving state (and its inverse model). In the foregoing drivetrain control apparatus, no distinction is made between the driving state and the driven state of the vehicle so the inverse operation of the drive torque from the required driving force may not be able to be accurately performed.